Infra red camera calibration

ABSTRACT

Described herein is a method and apparatus for calibrating an infrared camera at elevated temperatures using a reference surface. The method comprises selecting a normal well-fill condition for pixels in the camera in accordance with normal operating temperatures and stare time ( 30, 32, 34 ), using the normal well-fill condition to calculate a selected stare time/surface temperature combination for which selected non-uniform calibration coefficients are determined ( 36, 38 ), adjusting and re-adjusting the stare time/surface temperature to obtain adjusted and re-adjusted non-uniform calibration coefficients respectively ( 40, 42, 44, 46 ), and determining final non-uniform calibration coefficients for the camera using the selected, adjusted and re-adjusted non-uniform calibration coefficients ( 48 ).

The present invention relates to improvements in or relating to infraredcamera calibration, and is more particularly, although not exclusively,concerned with nonuniformity calibration.

It is known to perform internal two-point no uniformity calibration ininfra red cameras at ambient temperatures above 28° C. However, it hasbeen difficult to achieve such calibration due to the inability tocontrol the thermal reference surface temperature to reach the desiredset point for the operating temperature range of the camera based onselected stare time. The desired set point is typically 5° C. but theachievable reference surface temperature is approximately ambienttemperature less 25° C. This has the disadvantage that, in applicationswhere an infrared camera having a wide field of view is to be is used inambient temperatures equivalent to 55° C., the achievable referencesurface temperature is expected to be around 30° C. (55–25° C.). This issubstantially higher than both the desired set point and the specifiedscene temperatures equivalent to 10° C.

Whilst it is possible to modify the calibration technique so that cameracalibration can be carried out using the “best achievable” referencesurface temperature, a significant shortfall in the performance of thecamera is obtained relative to the performance which would be obtainedif the cameo calibration is performed using its ideal set pointtemperature. Moreover, if single-point calibration steps are performedat temperatures which are significantly different from those which arepresent in a background of a scene, the advantages and benefits of suchcalibration steps will be substantially lost as a result of thedifferences between the actual calibration temperatures and the desiredcalibration temperatures.

U.S. Pat. No. 6,127,679 discloses a thermal sensing system for observinga scene producing luminance comprising a reference IR LED for providinga calibrated predetermined luminance, an array of photon detectingelements, at least two position optical system for controllably focusingonto the array of detecting elements luminance from either an observedscene or from reference IR LED, a switching apparatus for controllablymoving said optical system between the two positions and a computer,responsive to the position of said optical system for calibratingsignals from the array of detecting elements resulting from sceneobservation, with signals resulting from reference IR LED.

It is therefore an object of the present invention to provide acalibration method which provides for the benefits of both single-pointand two-point calibration being substantially recovered whilst usingachievable reference surface temperatures rather than ideal referencesurface temperatures.

In accordance with one aspect of the present invention, there isprovided a method of calibrating an infrared detector using atemperature-adjustable reference surface (12) within it field of view,the method being characterized by the steps of:

-   -   a) controlling the temperature of the reference surface (12) to        a first surface temperature (T₁) and measuring the output of the        detector (18) over a first stare time (S₁), the first        temperature (T₁) and stare time (S₁) being selected so as to        achieve a predetermined first well-fill (W_(actual)) of the        detector pixels at the lowest possible temperature of the        reference surface (12);    -   b) processing the output measurements of the detector (18) to        obtain a first calibration coefficient (coeff_(cal1));    -   c) adjusting the temperature of the reference surface (12) to a        second surface temperature (T₂) and measuring the output of the        detector (18) over a second stare time period (S_(actual)), the        second temperature (T₂) and stare time (S_(actual)) being        selected so as to achieve a second predetermined well-fill (W₂)        of the detector pixels    -   d) processing the output measurements of the detector (18) to        obtain a second calibration coefficient (coeff_(cal2));    -   e) readjusting the temperature of the reference surface (12) to        a third surface temperature (T₃) and measuring the output of the        detector (18) over the first stare time period (S₁), the third        temperature (T₃) being selected so as to achieve the second        predetermined well-fill (W₂) of the detector pixels over the        first stare time period (S₁₎.    -   f) processing the output measurements of the detector (18) to        obtain a third calibration coefficient (coeff_(cal2)); and    -   g) adjusting the first calibration coefficient (coeff_(cal1)) on        the basis of the second and third calibration coefficients        (coeff_(cal2) coeff_(cal3)) obtained.

Preferably, the well-fill is selected to be approximately 50%, but itwill be appreciated that any other suitable well-fill value may be usedaccording to characteristics of the infrared camera being calibrated.

The second well-fill may be selected to be near 100% but again anysuitable well-fill value may be used depending an the characteristics ofthe infra red camera being calibrated.

According to the present invention step g) comprises determining thefinal non-uniform calibration coefficients in accordance with the sum ofthe selected and re-adjusted non-uniform calibration coefficients lessthe adjusted non-uniform calibration coefficients. Naturally, it will beunderstood that the way the non-uniform calibration coefficients foreach stare time/surface temperature combination are used for the finalcalibration may be varied in accordance with the characteristics of thecamera being calibrated.

In accordance with another aspect of the present invention, there isprovided apparatus for calibrating an infrared detector, the apparatuscomprising:

-   -   a temperature-controlled reference surface (12), the detector        (18) being located within the apparatus to view the reference        surface (12);    -   control means (14, 28) for controlling the temperature (T) of        the reference surface (12) and the stare time (S) of the        detector (12); and    -   processing means (28) for receiving output signals from the        detector (12) at first, second and third predetermined detector        stare time and reference surface temperature combinations and        for producing calibration coefficients corresponding to each of        the first, second and third predetermined detector stare time        and reference surface temperature combination, and for        determining final calibration coefficients for the detector from        the calibration coefficients determined for each stare time and        reference surface temperature combination, characterized in that    -   in the first stare time and reference surface temperature        combination, the first temperature (T₁) and stare time (S₁) are        selected so as to achieve a predetermined first well-fill        (W_(actual)) of the detector pixels at the lowest possible        temperature of the reference surface (12);    -   in the second stare time and reference surface temperature        combination, the second temperature (T₂) and stare time        (S_(actual)) are selected so as to achieve a second        predetermined well-fill (W₂) of the defector pixels; and

in the third stare time and reference surface temperature combination,the third temperature (T₃) is selected so as to achieve the secondpredetermined well-fill (W₂) of the detector pixels over the first staretime period (S₁).

For a better understanding of the present invention, reference will nowbe made, by way of example only, to the accompanying drawings in which:

FIG. 1 is a block diagram illustrating calibration apparatus inaccordance with the present invention; and

FIG. 2 is a flow diagram illustrating the calibration process inaccordance with the present invention.

In accordance with the present invention, a method will be describedwhich allows the benefits of both single-point and, by extension,two-point calibrations to be substantially recovered, whilst usingachievable reference surface temperatures instead of ideal referencesurface temperatures.

Turning initially to FIG. 1, a calibration apparatus 10 is shown. Theapparatus comprises a reference surface 12 whose temperature iscontrolled by a temperature control device 14. An infrared detectorarrangement 16 which is to be calibrated is positioned so that aninfrared detector 18 has the reference surface in its field-of-view. Thedetector has a cooling unit 20, a processor 22 and a memory unit 26 asis conventional. The processor 22 provides an output signal 24indicative of radiation incident on the detector 18.

The calibration apparatus 10 also comprises a controller 28 which isconnected to receive the output signal 24 from the detector arrangement16 and to provide control signals for the temperature control device 14and the memory unit 26.

When an infrared detector 18 is to be calibrated, connections asdescribed above are made so that for each temperature of the referencesurface 12, the output signal 24 is compared with the temperature in thecontroller 28. This provides calibration coefficients for a particulartemperature which are stored in memory unit 26 for use when the detector18 is in normal operation.

In accordance with the present invention, it is assumed that theprincipal sources of error which must be calibrated are pixel by pixelvariations in offset and scale factor. It is possible to calibrate foroffset and scale factor using a two-point, or possibly three-point,calibration technique.

It will readily be understood that an infrared detector or cameracomprises an array of pixels which collects the incident radiation, andthat each pixel tends to have its own characteristics which are definedas an offset value and scale factor.

For an ideal two-point calibration, the temperature of the referencesurface is first controlled to have a value close to, or at, theequivalent scene temperature operating point. A selected stare time ischosen and data is collected from the detector over the selected staretime. It will be appreciated that the detector receives a de-focussedimage of the reference surface over the stare time. The collected datais processed to determine correction values, for example, offset valuesof the pixels, at the equivalent scene temperature. The temperature ofthe reference surface is then altered to be different from the firsttemperature, that is, different from the equivalent scene temperature.

It will readily be understood that data at the first temperature isideally used to calibrate offsets and sensitivities under the sameradiance and stare time conditions under which the detector will be usedand gives the same “well-fill ” conditions that the detector will seefrom the scene. By the term “well-fill” is meant the amount of chargedeveloped by each pixel in response to the incident radiation.

However, as it is not possible to reduce the temperature of thereference surface to provide the required radiance value, the presentinvention provides for a selection of a combination of stare time andreference surface temperature which provides an equivalent well-fill.This requires a reduction in stare time to offset the increased spectralradiance at higher reference surface temperatures. The relationship isdictated by Planck's law and is non-linear.

The selection of a suitable combination of stare time and referencesurface temperature allows calibration at an equivalent well-fill to beperformed. However, the adequacy of such a calibration depends on theeffects of non-uniformity relating to changes in stare time either to benegligible or to be further calibrated out. Generally, it should beassumed that non-uniformity effects relating to changes in stare timeare not negligible and need to be estimated. This can be achieved byperforming two or more further calibrations at combinations of staretime and reference surface temperature which give equivalent well-fillvalues to one another.

In accordance with the present invention, a three-step calibrationprocess is provided which essentially comprises three combinations ofstare time and reference surface temperature to allow pixel offsets andscale factor values to be estimated for the well-fill and stare timecombination which will be used in practice. This is illustrated in FIG.2.

Suppose an infrared detector or imager is to be used with a stare time,S_(actual), (step 32) and a scene temperature, T_(actual), (step 30) togive a well-fill of W_(actual), thenW _(actual)=function(T _(actual,) S _(actual))

where W_(actual) has a typical value of 50% (step 34).

The first non-uniform calibration step, Cal 1, is performed at T₁ and S₁to give a well-fill W_(actual) at the coolest possible reference surfacetemperature (step 36). This means that S₁ is a shorter stare time thanthat used in practice, S_(actual).

The compensation (that is, the non-uniform calibration coefficients(step 38)) obtained is correct in terms of well-fill, but is in error ifthe detector has non-uniform sensitivities to the change in stare timefrom S_(actual) to S₁. These sensitivities can be measured andcompensated by two further calibration steps.

The second calibration step, Cal 2, is performed at an intermediatesurface temperature of T₂ and at a stare time of S_(actual). This givesa well-fill of W₂ which is, for example, near 100% well-fill (step 40).The non-uniform calibration coefficients are determined in step 42.

The third calibration step, Cal 3, is performed at a surface temperatureof T₃ using a stare time of S₁ to give a well-fill of W₂ (step 44) andthe non-uniform calibration coefficients are determined in step 46.

It will be appreciated that each of the second and third calibrationsteps provides non-uniform calibration coefficients, and thesenon-uniform calibration coefficients can be used to determine if anyadjustment is needed for the first non-uniform calibration step. In thisexample, the difference between the second and third non-uniformcalibration coefficients is used to effect adjustment of the firstnon-uniform calibration (step 48). However, it will be understood thatthe second and third calibration coefficients may be used in differentways to achieve adjustment of the first non-uniform calibrationcoefficient.

It is expected that all three calibration steps are performed atachievable temperatures and stare times.

The final non-uniform calibration coefficients, coeff_(final), can beexpressed as:coeff_(final)=coeff_(Cal1)+coeff_(Cal3)−coeff_(Cal2)

The method of the present invention is very simple in practice, althoughsome new surface temperature set points and corresponding stare timesneed to be calculated and tested to achieve equivalent well-fills.Moreover, the non-uniform calibration coefficients from three tests needto combined as described above.

The method of the present invention has the advantage that hottersurface temperatures can be utilised during calibration than isconventionally required.

1. A method of calibrating an infrared detector using atemperature-adjustable reference surface within its field of view, themethod being characterized by the steps of: a) controlling thetemperature of the reference surface to a first surface temperature andmeasuring the output of the detector over a first stare time, the firsttemperature and stare time being selected so as to achieve apredetermined first well-fill (Wactual) of the detector pixels at thelowest possible temperature of the reference surface; b) processing theoutput measurements of the detector to obtain a first calibrationcoefficient (coeff_(cal1)); c) adjusting the temperature of thereference surface to a second surface temperature and measuring theoutput of the detector over a second stare time period (Sactual), thesecond temperature and stare time (Sactual) being selected so as toachieve a second predetermined well-fill of the detector pixels d)processing the output measurements of the detector to obtain a secondcalibration coefficient (coeff_(cal2)); e) re-adjusting the temperatureof the reference surface to a third surface temperature and measuringthe output of the detector over the first stare time period, the thirdtemperature being selected so as to achieve the second predeterminedwell-fill of the detector pixels over the first stare time period; f)processing the output measurements of the detector to obtain a thirdcalibration coefficient (coeff_(cal3)); and g) adjusting the firstcalibration coefficient (coeff_(cal1)) on the basis of the second andthird calibration coefficients (coeff_(cal2), coeff_(cal3)) obtained. 2.A method according to claim 1, wherein the first predetermined well-fillof the pixels is approximately 50%.
 3. A method according to claim 1,wherein the second predetermined well-fill of the pixels isapproximately 100%.
 4. A method according to claim 1, wherein step g)comprises determining a final calibration coefficient in accordance withthe sum of the first (coeff_(cal1),) and third (coeff_(cal3))calibration coefficients less the second (coeff_(cal2),) calibrationcoefficient.
 5. Apparatus for calibrating an infrared detector, theapparatus comprising: a temperature-controlled reference surface, thedetector being located within the apparatus to view the referencesurface; control means for controlling the temperature of the referencesurface and the stare time of the detector; and processing means forreceiving output signals from the detector at first, second and thirdpredetermined detector stare time and reference surface temperaturecombinations and for producing calibration coefficients corresponding toeach of the first, second and third predetermined detector stare timeand reference surface temperature combination, and for determining finalcalibration coefficients for the detector from the calibrationcoefficients determined for each stare time and reference surfacetemperature combination, characterized in that in the first stare timeand reference surface temperature combination, the first temperature andstare time are selected so as to achieve a predetermined first well-fill(Wactual) of the detector pixels at the lowest possible temperature ofthe reference surface; in the second stare time and reference surfacetemperature combination, the second temperature and stare time (Sactual)are selected so as to achieve a second predetermined well-fill of thedetector pixels; and in the third stare time and reference surfacetemperature combination, the third temperature is selected so as toachieve the second predetermined well-fill of the detector pixels overthe first stare time period.
 6. Apparatus according to claim 5, whereinthe first predetermined well-fill of the pixels is approximately 50%. 7.Apparatus method according to claim 5, wherein the second predeterminedwell-fill of the pixels is approximately 100%.
 8. Apparatus according toclaim 5, wherein the final calibration coefficient is determined inaccordance with the sum of the first (coeff_(cal1),) and third(coeff_(cal3)) calibration coefficients less the second (coeff_(cal2),)calibration coefficient.